Indicate at build time which hwloc API version is being used.
This number is updated to (X<<16)+(Y<<8)+Z when a new release X.Y.Z actually modifies the API.
Users may check for available features at build time using this number (see How do I handle API changes?).
- This should not be confused with HWLOC_VERSION, the library version. Two stable releases of the same series usually have the same HWLOC_API_VERSION even if their HWLOC_VERSION are different.